Range calls f sequentially for each key and value present in the map.
If f returns false, range stops the iteration.

Range does not necessarily correspond to any consistent snapshot of the Map's
contents: no key will be visited more than once, but if the value for any key
is stored or deleted concurrently, Range may reflect any mapping for that key
from any point during the Range call.

Range may be O(N) with the number of elements in the map even if f returns
false after a constant number of calls.